According to recent motion video encoding techniques, each picture is encoded using different coding modes having different compression efficiencies. For example, a picture is encoded as an intra-predicted picture (I picture), which is a picture encoded using only intra-picture prediction, or as a unidirectionally predicted picture (P picture) or bidirectinally predicted picture (B picture), which is a picture encoded using inter-picture prediction. Since the amount of code that occurs differs according to the type of the picture or the complexity of the image, the amount of code tends to vary from one time to the next. As a result, to transmit a data stream containing encoded motion video at a constant transmission rate, a transmit buffer and a receive buffer for buffering the stream have to be provided. The delay caused by this buffering is one of the factors that cause a display delay at the motion video decoder end. If the buffer size is reduced, the buffer delay can be reduced at the motion video decoder end, though the picture quality tends to degrade, because the degree of freedom in distributing the amount of code among the pictures decreases.
International motion video coding standards currently in widespread use, such as MPEG-2 Video (ISO/IEC 13818-2/ITU-T H.262, hereinafter referred to as MPEG-2) and MPEG-4 AVC/H.264 (ISO/IEC 14496-10/ITU-T H.264, hereinafter referred to as H.264), provide for the operation of a stream receive buffer in an idealized decoder referred to as the video buffering verifier (VBV) or the coded picture buffer (CPB), respectively. A motion video encoder has to control the amount of code so that the receive buffer in the idealized decoder neither overflows nor underflows. It is stipulated that the idealized decoder performs instantaneous decoding that takes zero time to decode. Patent document 1, for example, discloses a moving video encoder control method concerning the VBV.